1. Field of the Invention
This invention relates to a polycarbonate copolymer and an organic optical component made from the polycarbonate. The polycarbonate copolymer of this invention has excellent transparency, heat resistance and mechanical properties and a low birefringence, as well as good melt fluidity and moldability. The polycarbonate copolymer of this invention is useful for an optical component; typically a substrate for an optical disk, a pick-up lens and an optical fiber.
2. Description of the Related Art
A polycarbonate is widely used as an engineering plastic in the fields of automobile, electronics and optics. Currently, a polycarbonate widely used is generally prepared from 2,2'-bis(4'-hydroxyphenyl)propane(hereinafter, referred to as bisphenol A) and a halogenated carbonyl compound such as phosgene. A polycarbonate from bisphenol A is a resin having well-balanced properties such as transparency, heat resistance, a lower moisture permeability, shock resistance and dimensional stability. It is, therefore, widely used, and in particular has been recently used in the field of optical components such as a substrate for an optical disk.
In an optical disk used as an information recording medium, a laser beam passes through the disk body. Thus, the disk is required to be, of course, transparent, and is strongly required to be optically isotropic for reducing reading errors of information.
However, for example, when using a polycarbonate from bisphenol A, there occurs a problem that a residual stress generated by some factors such as thermal stress, molecular orientation and volume variation near a glass-transition temperature generated by cooling and fluidizing processes of a resin during casting a disk substrate, may cause a birefringence when a laser beam passes through the disk substrate. Large optical anisotropy due to the birefringence may become a fatal defect for an optical component such as an optical disk substrate because it may cause significant problems such as reading errors of a recorded information. Furthermore, due to having a higher melt viscosity, the polycarbonate exhibits poor moldability, e.g., it may be quite difficult to provide a disk substrate with a narrower track pitch and a shorter pit diameter for improving a recording density by injection molding.
A variety of new polymers have been disclosed for solving the above problems, e.g., a polycarbonate from spirobiindanol alone and a polycarbonate copolymer from spirobiindanol and bisphenol A (for example, U.S. Pat. No. 4,950,731).
Although having a low birefringence, the former polycarbonate is practically problematic due to its poor transparency and mechanical strength; for example, it may have cracks during molding. The latter polycarbonate, increase of bisphenol A ratio improves transparency and mechanical strength, but increases the birefringence, which limits its applications as an optical component for, e.g., the above information recording medium.
A polycarbonate copolymer (alternating copolymer) from spirobiindanol and bisphenols of specific structure has been disclosed in JP-A HEI7-330884. However optical properties of the polycarbonate such as birefringence is not mentioned. The use of the polycarbonate for optical components such as information recording medium is practically problematic due to its high birefringence.
Furthermore, compared with a polycarbonate from the existing bisphenol A, these polycarbonates does not have a practically sufficient melt fluidity for providing a disk substrate for an information recording medium which is required to have the above precise processability, by injection molding. Thus, it has been strongly desired to solve these problems.